U.S. Pat. Nos. 3,528,264 and 3,922,884 disclose such couplings and they comprise an input member connectible to a driving shaft and an output member connectible to a driven shaft, said members being interconnected by flexible coupling elements which are deformable in the plane of rotation by torque transmitted by the coupling and by centrifugal force acting on the coupling elements such that over a range of speeds and torques one member is relatively rotatable with respect to the other.
It has been theoretically proven and experimentally confirmed that if a coupling having zero torsional stiffness be connected between two parts of a rotating system and be capable of transmitting a torque, then, if one part of the system be subjected to torques fluctuating even violently above and below a mean torque, the torque transmitted to the other part of the system will be the mean torque completely devoid of any fluctuations above or below the mean torque.
It has also been proven and experimentally confirmed that if the ratio of the torsional stiffness of a coupling to a mean torque being transmitted at any time (referred to as the "S/T ratio") has a low value, then, if the one part of the rotating system be subjected to torque fluctuations of a given value above and below the mean value, the value of the torque fluctuations transmitted through the coupling to the other part of the system will be almost directly proportional to the S/T ratio. Thus, if the S/T ratio at the time, speed, and mean torque considered be zero, there will be no transmitted torque fluctuations, irrespective of the fluctuations on the transmitting side. If on the other hand the S/T ratio has a relatively high value--say 50 or more--the coupling will have little effect in reducing the transmitted fluctuations.
At intermediate values the coupling will transmit a proportion of the fluctuations imposed on it.
The coupling characteristics of the coupling described in our U.S. Pat. No. 3,922,884 are such that it maintains a minimum S/T ratio whilst running along its design power/speed line which equates to the well known power cube law relationship. The design line can be varied by altering the geometrical structure of the coupling, but, deviation from this line at a given speed will result in an increase in S/T ratio. This ratio will remain low i.e. below 10 for a range of torques at any given speed.
It is therefore an object of this invention to provide a coupling which has an S/T ratio which is low over a wider range of torques and speeds than has hitherto been thought possible with existing couplings. Alternatively, the invention provides a coupling which has lower S/T ratios, over a comparable torque and speed range, than has hitherto been thought possible with existing couplings.